The present invention relates generally to brazing, and, more specifically, to wetting balances for measuring wetting force during brazing.
Soldering and brazing are related processes used for bonding together metal components. Soldering occurs at relatively low temperature for typically bonding together copper or brass parts, and brazing requires substantially higher temperatures for bonding together high strength materials including stainless steel and various superalloys typically used in manufacturing gas turbine engines, including nickel based superalloys.
Various brazing materials are tailored to the specific material of the parts being brazed for effecting high strength joints therewith. However, an effective braze joint requires wetting of the molten braze material with the surface of the part so that upon cooling and solidification thereof a strong integral metallurgical bond is created.
Wetting of solder with copper parts generally occurs practically instantaneously. Wetting of typical braze materials with corresponding parts may occur practically instantaneously or may require a considerable time exceeding tens of seconds, and in some cases wetting may not occur at all irrespective of the amount of time permitted therefor.
The wetting time of a particular braze material with a corresponding part requiring brazing is a primary parameter in the brazing process. Whether brazing is effected manually or automatically, sufficient time must be provided to ensure proper wetting for obtaining a full strength braze joint.
Wetting performance of solder and braze materials is typically evaluated in commercially available wetting balances. A typical wetting balance suspends a specimen from a weighing device such as a micro-balance or load cell, and a crucible containing molten solder or braze material is lifted for immersing the bottom portion of the specimen into a molten solder or braze pool to a preferred depth. The molten solder or braze pool applies a force to the specimen which varies depending upon the degree of wetting or non-wetting thereof.
In wetting, the molten pool adheres to the specimen and creates a rising positive meniscus having a corresponding wetting angle less than 90.degree.. In non-wetting, the molten pool does not adhere to the specimen and effects an opposite depression or negative meniscus with a correspondingly different wetting angle greater than 90.degree..
By accurately measuring the force applied to the specimen during immersion in the molten pool, the wetting angle and wetting force may be analytically derived. Accordingly, the wetting performance of various solder or braze materials may be quantitatively evaluated.
However, one form of a wetting balance uses a precision micro-balance beam for suspending the specimen and measuring the applied force from the molten pool. The balance beam measures force precisely, yet is relatively slow in operation for balancing the applied force. And, the specimen correspondingly changes immersion depth as the balance beam equilibrates.
Another type of wetting balance includes a conventional load cell from which the specimen is suspended. The load cell, although not as precise as the balance beam, measures applied force substantially instantaneously. And, the specimen is maintained at a fixed elevation which does not affect the immersion depth in the molten pool.
In both wetting balance types, the specimen is partially immersed in the molten pool, and the immersion depth must be accurately determined for use in accurately determining the wetting angle and wetting force. Immersion depth is typically separately measured by optical observation.
Accordingly, it is desired to provide a wetting balance having improved accuracy and ease of use in measuring applied force and determining corresponding wetting force.